The United States is a very wealthy nation. Addressing the health issues of the United States is a very specialized task. Improving the healthcare system of the United States takes a very deliberate effort. The United States system of healthcare is so advanced that for every percentage of improvement you want to make of that system, you have to successively put large amounts of funding.
Compared to the United States System of Healthcare, Nepal's system is relatively underdeveloped. So it has a lot more room for improvement with relatively less effort and resources. There are factors that contribute towards health in Nepal that someone in the United States wouldn't even consider. The United States is focussed on a lot more specific issues in relation to healthcare.
For example, I was looking at a project in Nepal. The project addresses issues that someone living in the United States wouldn't even consider to be health issues. Clean drinking water? Clean bathrooms? Smokeless stoves?
All these small details contributes considerably towards improving the health of the Nepali people.